In electrostatography, a latent electrostatic image is formed on an insulating substrate such as a photoconductor. This image can be formed by a variety of methods including the use of light of visible or non-visible (e.g., x-ray) wavelength, or electronically by electrographic recording. Imagewise charge patterns can also be made by other electrostatographic means such as ionography and ion projection. The latent electrostatic image can be developed (i.e., made visible) by the application of a developer containing charged colored particles, called toner particles, to the latent image. The charged toner particles adhere to the latent image in proportion to the imagewise potential difference. The developer can be either a dry powder or dispersed toner particles in an electrically insulating liquid.
While it is not necessary for toners used in many electrostatographic processes to be fluorescent, fluorescent toners are very advantageous if the toners are to be used in xeroradiography. (See, for example, U.S. Pat. Nos. 2,817,767 and 2,856,535.) Briefly, in xeroradiography, a charged photoconductor is exposed to x-rays which has passed through an object (e.g., a portion of a human body) of which one wishes to obtain an x-ray image, forming a latent electrostatic x-ray image on the photoconductor. The latent image is toned with a fluorescent toner and the toned image is exposed to light. The image fluoresces in proportion to the amount of fluorescent toner that is present and can be photographed or used to expose a photoconductor for xerographic copying. The use of a fluorescent dye in this process enhances the contrast of the image and reduces the intensity of the x-rays needed to form the image.
Fluorescent dyes do not themselves have properties that toners must have to develop an electrostatic latent image. Thus, a fluorescent toner must be made by incorporating a fluorescent dye into a polymeric binder (with other components). When certain fluorescent dyes are directly mixed with suitable binders by melt-compounding and grinding, the most common method of preparing a toner, the electrical properties of the toner are disturbed, so that images formed with the toner are of poor quality. Moreover, if the toner formulation is optimized for a particular dye, the formulation may not be optimum for a different dye.
If the fluorescent dye is not melt-compounded with the binder, but is merely mixed with it, the dye may not fluoresce, or may not fluoresce well, because it is not in solid solution in the binder. While a solid solution of the dye can be made by forming a solution of the binder and the dye and removing the solvent, this procedure cannot be used when the binder is insoluble or is not soluble in commonly used or non-toxic solvents. Also, toners prepared in this manner may have inferior properties, such as unstable charging characteristics, compared to toners prepared by melt-compounding.
In addition to maintaining the electrical properties of the toner and the fluorescence of the dye, a procedure for making a fluorescent toner should also be capable of producing a toner of small particle size (e.g., less than a micrometer for liquid toners and less than 20 micrometers for dry toners), since high resolution images require smaller sized toners. If a pigment particle is to be incorporated into a toner particle without substantially altering its characteristics, the pigment particle must be small enough, relative to the toner particle, so that its properties do not significantly perturb the properties of the particle as a whole. A good procedure for making a fluorescent toner should therefore make or use small pigment particles (e.g., less than one micrometer).
In commonly assigned U.S. Pat. No. 4,865,937, issued Sep. 12, 1989, inventors D. Santilli and J. W. May, there is disclosed method of making a fluorescent toner based on the discovery that fluorescent dyes can be incorporated into toners without adversely affecting the electrical properties of the toner or the fluorescence of the dye if the dye is first made into a fluorescent pigment, and then the pigment is melt-compounded with the toner binder. (Herein, "pigment" means a fluorescent phase, separate from the binder, in which a fluorescent dye is dissolved in an organic polymer. "Fluorescent" means emitting light after excitation and may include luminescent, phosphorescent, and scintillating.) The pigment is prepared by mixing a solution of the dye and an organic polymer with a non-solvent in the presence of a dispersant. This procedure precipitates submicrometer sized pigment particles, which can be incorporated into small toner particles without substantially altering the electrical surface characteristics of the toner particles.
As disclosed in the latter patent, an electrographic liquid developer is made by diluting the fluorescent toner in a developer vehicle having a low flash point and high evaporation rate. This liquid developer may not be appropriate in applications where it is desirable to keep the liquid toned image wet for a period of time for further image processing. Moreover, a low flash point, high evaporation rate liquid developer imposes safety considerations during storage and transportation, increases the cost of disposal of spent developer liquid and increases the cost and complexity of manufacturing electrographic equipment using such liquid developer. In addition, to meet government regulatory requirements for managing emissions of organic vapors generated by evaporation of a low flash point, high evaporation rate liquid developer is expensive.